1. Field of the Invention
The present invention relates to an active noise cancellation system, and more specifically relates to a system for emitting or outputting a signal (sound or vibration) that cancels out the vibration or noise (or vibration-induced noise) in the passenger compartment of a vehicle, the cabin of an aircraft, or the like, and controlling that signal so that vibration or noise is effectively canceled or minimized by the resultant interference.
2. Description of the Related Art
Systems have been proposed as active noise cancellation systems whereby a noise-canceling signal is emitted or outputted from a speaker or the like by using a digital signal processing technique, and the noise at a listening position (evaluation point) at which a microphone or the like is installed is reduced (see Japanese Domestic Republication No. 1-501344 that is corresponding to PCT/GB87/00706 (FIG. 1 and others) and Japanese Laid-Open Patent Application No. 6-332477 (FIG. 1 and others)).
The technique described in Japanese Domestic Republication No. 1-501344 is configured such that a plurality of speakers as canceling signal emitters and microphones as error signal detectors are disposed in the passenger compartment of a vehicle, the cabin of an aircraft, or another enclosed space, and noise is reduced in the entire enclosed space of the vehicle passenger compartment or the like.
Specifically, this type of noise cancellation system essentially employs feedforward control using an adaptive filter to emit a signal from a speaker so as to minimize an error signal that indicates residual vibration or noise due to the interference between a noise and the canceling signal in the mounting position of the microphone, and therefore has the drawback of being incapable of adequately reducing noise that is located away from the microphone.
The technique described in Japanese Domestic Republication No. 1-501344 is therefore designed such that the control area in which noise can be reduced is extended from a point to a space, and noise can be reduced throughout an enclosed area by installing a plurality of microphones and performing control such that the summation of the error signals detected by each microphone is minimized.